As generally known in the art, a lithium polymer battery is a battery having an electrode assembly comprising a separator positioned between positive and negative electrode plates. The separator performs the role of the electrolyte, namely, it acts as a medium of ion conduction. In addition, the separator acts as a medium for separation. The separator improves ion conductivity and comprises a gel-type polymer electrolyte, which is manufactured by impregnating a polymer with an electrolyte. In addition to improved ion conductivity, the gel-type polymer electrolyte bonds well to electrodes, has good mechanical properties and is easy to manufacture. A polyvinylidene fluoride (PVDF) based electrolyte available from Bellcore Company is one representative gel-type polymer electrolyte and is manufactured by mixing a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), a plasticizer and an inorganic additive to form a film, impregnating the film with an electrolyte, and causing the film to gel.
Lithium polymer batteries and lithium ion batteries differ in several respects. First, unlike lithium ion batteries, lithium polymer batteries can have a plate structure and need not be wound. Therefore, lithium polymer batteries can have electrode assemblies that comprise several laminated plates and can be manufactured to have a square structure. Second, in lithium polymer batteries, the electrolyte is injected into a completely integrated cell and rarely leaks out. Third, the plate structure of lithium polymer batteries makes it unnecessary to apply pressure to form the square shape. Therefore, a thin, flexible pouch can be used as the battery sheath, instead of a hard square or cylindrical can.
When a flexible pouch is used as the battery sheath, the thickness is substantially less than the thickness of a battery using a can as the battery sheath. Therefore, more electrode assemblies can be contained within the same volume. This remarkably increases battery capacity. The flexibility of the pouch type battery sheath enables the battery to be easily manufactured in any desired shape, and enables easy mounting of the battery on various electronic appliances. However, pouch type battery sheaths do not radiate heat well, thereby reducing the life of the battery. Specifically, pouch type battery sheaths generally comprise nylon or polyethylene terephthalate (PET) on their surfaces. These materials decrease radiation performance and cannot effectively withstand the heat generated during charging and discharging. Furthermore, the amount of discharge increases as temperature rises, which drastically reduces battery life.
When the temperature of the battery rises above a threshold level, the electrode assembly or the electrolyte decomposes, generating a large amount of gas. This gas generation increases the internal pressure of the battery and causes the battery to swell, seriously decreasing battery reliability.
Lithium polymer batteries can be equipped with positive temperature coefficient (PTC) devices. The resistance of such a PTC device increases relative to temperature, and interrupts charging currents upon increases in temperature. Such a PTC device can be positioned on the protective circuit module itself or on the connection between the protective circuit module and the electrode assembly. The PTC device reduces or interrupts currents flowing through the device when the temperature of the battery increases. Upon reduction or interruption of the currents, heating and temperature increases in the battery generally stop. A portion of the PTC device contacts the battery sheath, enabling it to respond to the temperature of the battery. However, the battery sheath comprises nylon or PET on its surface, which decreases radiation performance as noted above, and heat generated inside the sheath is not quickly transmitted to the PTC device. Accordingly, the PTC device fails to respond quickly to the rising temperature of the battery, seriously decreasing battery reliability.
In addition, conventional battery sheaths have low mechanical strength and are very vulnerable to external impact. For example, a puncture is easily formed if the sheath is pierced with a sharp object such as a needle or nail. Also, the sheath is easily torn if bitten by a pet. Furthermore, penetration of the sheath with a sharp object can cause a short circuit if the object contacts the internal electrode assembly. As a result, the battery may catch fire or explode.